Self-cleaning spark plug

ABSTRACT

An improved spark plug adapted for use in rotary engines in order to eliminate misfiring in the engine caused by shorting of the electrodes as a result of the build up of oils, carbon and other products of combustion on the electrodes, comprising an insulator and center electrode assembled in conventional manner within a metal shell, the lower end of which cooperates with the center electrode to form a sparking gap, the gap being located in the restricted cross-section outlet of a combustion cavity formed within the lower end of the shell between the inner wall surface of the shell and the outer surface of the insulator, the combustion cavity having a volume of from 1-3% of the volume of the engine combustion chamber and the outlet area being such as to produce an exiting combustion gas velocity from the cavity upon ignition of the fuel-air mixture therein such as will maintain the electrodes in a clean condition.

United States Patent 1 1 3, Louzecky or. 26, 197% SELF-CLEANING SPARKPLUG Primary Examiner-Alfred L. Brody [75] Inventor: Paul J. Louzecky,Troy, Mich. Attorney Agent or pirmhsldney Carter [73] Assigne'e: GeneralMotors Corporation, A

Detroit, Mich. [571 ABSCT [22] F] d Oct 2 1972 An improved spark plugadapted for use in rotary en- App]. No.: 294,476

gines in order to eliminate misfiring in the engine caused by shortingof the electrodes as a result of the build up of oils, carbon and otherproducts of combustion on the electrodes, comprising an insulator andcenter electrode assembled in conventional manner within a metal shell,the lower end of which cooperates with the center electrode to form asparking gap, the gap being located in the restricted cross-sectionoutlet of a combustion cavity formed within the lower end of the shellbetween the inner Wall surface of the shell and the outer surface of theinsulator, the combustion cavity having a volume of from 13% of thevolume of the engine combustion chamber and the outlet area being suchas to produce an exiting combustion gas velocity from the cavity uponignition of the fuel-air mixture therein such as will maintain theelectrodes in a clean condition.

1 Claim, 1 Drawing Figure 1 SELF-CLEANING SPARK PLUG the lower endthereof between the shell and the insulator is not new as shown by thepatent to G. F. Wright et al. No. 2,702,537 dated Feb. 22, 1955 and thepatent to Testerini No. 3,215,131 dated Nov. 2, 1965. Neither is the useof a conventional type spark plug in a rotary engine new as shown by thepatent to Shibagaki et al. No. 3,597,648 dated Aug. 3, 1971. Each ofthese pa tents, however, has an entirely different structure andoperation than that disclosed and claimed by applicant.

In accordance with my invention, a spark plug of the self-cleaning typeis provided. The electrodes are maintained in a clean condition by theself-cleaning action of the plug resulting from the hot scrubbing of theelectrodes by the ignition of the fuel-air mixture within the lower endof the plug upon each sparking cycle of the plug, the hot combustiongases exiting from the plug and sweeping over the electrodes positionedin the plug exit.

The features and operation of my invention are apparent from thefollowing description taken in conjunction with the accompanying drawingin which there is shown a longitudinal half-section view of the plugtaken through the centerbore axis as installed in a radial engine shownin partial view.

As shown in the drawing, the spark plug 1 comprises a metal shell 3 inwhich there is positioned in sealed relationship therewith a ceramicinsulator 5 having a centerbore 7 axially through the center thereof. Acenter electrode 9 is positioned within the centerbore with its sparkingend portion 11 extending beyond the end of the insulator. As shown, thespark plug assembly is threaded into the housing 13 of the rotary enginewith the plane of the end of the spark plug being limited to the planeof the surface of the combustion chamber or trochoid surface in the areaof the spark plug aperture formed in the housing 13 in order to avoidany interference with the rotor apex seals 14 in the engine.

As can be readily understood from consideration of the mode of operationof a rotary engine, the seals provided in the ends of the engine rotorin order to seal the several portions of the engine cavity from eachother through each cycle of rotation, act to sweep, or shovellubricating oil and solid combustion products into the lower end of thespark plug shell, thus tending to quickly foul the electrodes withresultant misfiring of the plug. In order to control this misfiringtendency, applicant has designed his plug in order to provide acombustion cavity 15 within the lower end of the spark plug, cavity 15being formed between the small diameter lower end 17 of the insulatorand the radially inwardly tapering surface 19 of the shell, the radiusof the inner surface of the shell decreasing progressively from theinner end 16 of the cavity 15 to the restricted crosssection circularoutlet end 21. The portions defining the combustion cavity 15 are soproportionedas to produce a combustion cavity volume of from about 1-3percent of the volume of the engine combustion chamher.

As is shown, the sparking end portion 11 of the center electrode ispositioned within the restricted crosssection circular outlet end 21 ofthe shell 3 with which it cooperates to form a spark gap. As shown inthe pre ferred embodiment, the spark gap is annular in shape. However,the gap may be alternatively formed as in the conventional automotivepin type spark plug by the provision of a ground electrode secured tothe end of the shell and projecting toward but spaced from the side ofthe center electrode in order to maintain the spark gap in therestricted cross-section outlet from the combustion cavity.

It is the purpose of my invention that the electrodes be positioned inthe restricted cross-section outlet 21 of the spark plug in order thatthe products of combustion from within the combustion cavity 15 sweep bythe electrodes through the gap area in order to clean the oil andcombustion products from the electrodes.

As is shown in the drawing, in the preferred embodiment of my spark plugthe small diameter lower end 17 of the insulator projects partially intoa tubular extension portion 23 of the shell 3, the tubular extensionforming the restricted cross-section circular outlet end of the plug andof the combustion cavity 15. By so designing the parts, an annularventuri-like throat is formed through which the combustion gases fromthe cavity 15 pass at a relatively high velocity. The velocity of thegases iassingthrough the exit and through the sparkgap is from about900m l000 feet per second at its peak upon ignition of the combustiblegases within the cavity 15. The design of the combustion cavity is noproblem to those skilled in the art in view of my teachings as set forthabove. By way of example, a suitable sgk plugfor use in a rotaryenginelnwhich the combustion chamber volume is about 30 cubic inchesincludes acombustion cavity 15 having a volume of 060 cubic inch or 2 percent ofthe engine combustion chamber volume. The annular cross-section area ofthe restricted outlet at the exit end of the cavity is approximately0.04 sq. in. based on the tubular extension portion 23 having an insidediameter of about 0.28 inch and the small diameter portion 117 of theinsulator extending into the tubular extension 23 having a diameter ofabout 0.16 inch, the engine compression ratio being from about 8 to9.521. The width of the spark gap is similar to that in conventionalautomotive type plugs and is about 0.035 inch.

Though my invention has been described to include a tapered outersurface portion on the lower end of the spark plug shell, it is apparentthat the outer surface might equally well be cylindrical as isconventional. Equivalents as regards other features will also beapparent to those skilled in the art and such equivalents are intendedto be within the scope of the claims which follow.

What is claimed is:

l. A self-cleaning spark plug comprising a metal shell having a tubularfiring end portion of circular crosssection, a ceramic insulator securedtherein in gas-tight relationship therewith and having a centerboretherethrough, the surface of the lower portion of said insulator beingtapered and spaced apart from said shell and of circular cross-sectionthroughout its length with the firing end thereof extending into saidtubular end portion concentrically and coaxially therewith to form anpositioned in said centerbore, said firing end portion cooperating withsaid center electrode to form an annular spark gap in said outletadjacent the venturi-like throat, the cross-sectional area of saidannular spark gap being such as to produce an initial velocity ofcombustion gases out of said cavity and through said spark gap of fromabout 900 to 1000 feet per second in order to keep the electrodesscubbed clean.

1. A self-cleaninG spark plug comprising a metal shell having a tubularfiring end portion of circular cross-section, a ceramic insulatorsecured therein in gas-tight relationship therewith and having acenterbore therethrough, the surface of the lower portion of saidinsulator being tapered and spaced apart from said shell and of circularcross-section throughout its length with the firing end thereofextending into said tubular end portion concentrically and coaxiallytherewith to form an annular venturi-like throat therebetween of uniformwidth, the inner wall surface of said shell and the surface of the lowerportion of said insulator forming a combustion cavity of from about 1 to3 percent of the volume of the engine combustion chamber volume, saidinner wall surface of said shell defining said combustion cavity beingtapered radially inwardly to said tubular end portion to form arestricted cross-section outlet at the exit end of said cavity, a centerelectrode positioned in said centerbore, said firing end portioncooperating with said center electrode to form an annular spark gap insaid outlet adjacent the venturilike throat, the cross-sectional area ofsaid annular spark gap being such as to produce an initial velocity ofcombustion gases out of said cavity and through said spark gap of fromabout 900 to 1000 feet per second in order to keep the electrodesscubbed clean.